PALAIOS, 2010, v. 25, p. 167–182 Research Article DOI: 10.2110/palo.2009.p09-054r THE DECAPODA (CRUSTACEA) AS PREDATORS ON MOLLUSCA THROUGH GEOLOGIC TIME CARRIE E. SCHWEITZER1* and RODNEY M. FELDMANN 2 1Department of Geology, Kent State University Stark Campus, 6000 Frank Ave. NW, North Canton, Ohio 44720, USA; 2Department of Geology, Kent State University, Kent, Ohio 44242, USA e-mail: [email protected] ABSTRACT decapods and lineages known to have modern congeners that are durophagous. The relationship between predator and prey is a persistent theme in There are five major means by which decapods crush shells or eat marine paleontology. Herein we focus on the decapod Crustacea, the shelled prey that, as they require specialization of the appendages (see shrimps, lobsters, and crabs, and their role as predators on the Mollusca Lau, 1987 on extant forms), might be recorded in the body fossil record. through geologic time. Five major means by which decapods crush shells There are also several types of feeding that may only chip the shells or or eat shelled prey might be recorded in the body-fossil record, as they simply pry them open, which are often not apparent in the fossil record. require specialization of the appendages. These include use of (1) All types of feeding on shelled organisms must affect evolution of the heterochelous first pereiopods, (2) molariform teeth on the fingers of shelled organism, based on recent studies that illustrate how predation the chelae, (3) a curved proximal tooth on the movable finger of the chela, takes precedence over other aspects of the environment in driving (4) calcified mandibles, and (5) flattened pereiopods (walking legs). evolution (Stanley, 2008). Studies of predation, evolution, and Decapods have had adaptations for durophagous predation on mollusks escalation should, therefore, consider all decapods that have adapta- since the early Triassic. Durophagous adaptations had appeared among tions for durophagy or that have adaptations for durophagy in extant multiple clades by the Late Cretaceous. The myriad means by which forms, not just those that leave easily recognized marks on shells. decapods prey upon Mollusca, and the multiple uses for which pereiopods There are some a priori assumptions in all studies related to decapod and other appendages are adapted, suggests that predation studies should predation. One assumption is that the thickening of molluscan shells incorporate more decapod types and more types of predation when and the development of more ornate ornamentation on shells is an examining predation as a driver of evolution. evolutionary adaptation to survive increased durophagous predation (Vermeij, 1977a, 1987). A second assumption is that strong chelae INTRODUCTION armed with stout denticles evolved as a predatory device, rather than for any other function (Lee, 1995; Schenk and Wainwright, 2001). A One of the persistent themes in the literature on marine paleontology third assumption is that shell-crushing appendages or other parts were has been the effect of predation by decapod crustaceans on the used in a way similar to present-today representatives in the Decapoda. evolutionary biology of their prey—commonly, the Mollusca. This type Many adaptations facilitating capture and predation of Mollusca could of predation is called durophagy, defined as the consumption of prey also facilitate predation on such shelled prey as Bryozoa and with a hard shell or skeleton, most often by crushing or drilling Brachiopoda. (Aronson, 2001). Pelecypods and gastropods have evolved a variety of strategies to reduce the effects of decapod predation, including the Systematic Definitions evolution of thicker shells and various elements of ornamentation. This relationship has been considered part of the Mesozoic Marine Various groups of the Decapoda are defined in the following section. Revolution (MMR) (Vermeij, 1977a, 1987) and other evolutionary Definitions are arranged alphabetically for ease of use. events (Walker and Brett, 2002). Typically, the interpretation of the Anomura.—A diverse group in which the abdomen may be carried predator-prey relationship has been viewed heavily from the perspective behind or underneath the cephalothorax (body) or in a shell or other of the response of the prey species. Less attention has been placed on structure, such as sea anemones, and in which the last pair of the multiple strategies used by the predators, their concomitant pereiopods is generally reduced and the abdomen may be asymmetrical. morphological expression, and the fact that not all predation may be This group includes the king crabs, snow crabs, mole crabs, hermit obvious on shell remains in the fossil record. crabs, and squat lobsters. Nearly all decapods are categorized as generalists based on their Brachyura.—The true crabs, an apparently monophyletic lineage of dietary preference, although a few have an amazingly limited diet. decapod crustaceans holding a symmetrical abdomen entirely or mostly From the standpoint of the paleontological record, it is possible to infer under the cephalothorax and usually with five pairs of pereiopods, the the manner of food capture and manipulation based on the morphology first of which is chelate and the second through fifth are generally of the decapod. Thus, various styles of predation may be suggested even achelate, although some have chelate or pseudochelate fifth pereiopods. under circumstances in which little or no trace of predation is evident Crab.—A general term for a decapod crustacean with an abdomen on putative prey species. Approaching the predator-prey relationship that is held entirely or mostly under the cephalothorax. This group from this standpoint may reveal potential interactions that cannot be includes brachyurans and anomurans. deduced simply by studying effects on prey. Heterotrematous Crabs.—Brachyurans in which the females possess The purposes of this paper are to (1) review the major documented genital openings on the sternum (ventral surface) and the males possess adaptations for durophagy in the Decapoda and (2) document the first genital openings on the coxae of the fifth pereiopods. This group may appearance of adaptations known to facilitate durophagy in modern be monophyletic and is considered more derived than many other brachyurans; studies are ongoing (De Grave et al., 2009). Lobster.—A general term for a decapod crustacean in which the * Corresponding author. carapace and abdomen are well-calcified and the abdomen extends Copyright G 2010, SEPM (Society for Sedimentary Geology) 0883-1351/10/0025-0167/$3.00 168 SCHWEITZER AND FELDMANN PALAIOS posteriorly from the cephalothorax. There are chelate (Astacidea, some Lobsters that have large, usually heterochelous, chelae are found Palinura), achelate (Palinura), and pseudochelate (Glypheoidea) chiefly in the superfamilies Nephropoidea and Enoplometopoidea lobsters. Extant achelate lobsters are commonly called spiny lobsters. within the Astacidea (see Supplementary Data1). The Nephropoidea Podotrematous Crabs.—Brachyurans in which both males and includes several extinct and extant lineages, and their claws tend to be females possess genital openings on the coxae of the pereiopods (fifth strongly heterochelous. Variation within the Nephropoidea is so great and third, respectively). The group is probably polyphyletic; studies are that it is difficult to generalize on the manner in which food is obtained ongoing (De Grave et al., 2009). This group is considered to embrace (Wassenberg and Hill, 1989). For example, a few taxa within the the most primitive brachyurans. Thaumastochelidae have extremely specialized claws, hypothesized to Thoracotrematous Crabs.—Brachyurans in which the both the males be adapted for raking the seafloor and grasping soft-bodied prey. Most and females possess genital openings on the sternum (ventral surface). nephropoids, however, have claws similar to those of the American The group may be monophyletic; studies are ongoing (De Grave et al., lobster. 2009). This group is considered to contain the most derived In the American lobster, Homarus americanus H. Milne Edwards, brachyurans. 1837, a member of the Nephropidae, the first pereiopods are heterochelous. These animals have a cylindrical carapace and carry Institutional Abbreviations the claws anterior to the carapace. We have observed that these animals typically move forward and backward rather than sideways so it is Listed below are the institutional abbreviations used in this paper. probable that they approach prey straight on. Prey is attacked by the BMNH: The Natural History Museum, London, UK. crusher claw and the majority of breakage accomplished by the UKKSU D: Decapoda Collection, Department of Geology, Kent chelipeds. Manipulation by the minor claw and mastication by the State University, Kent, Ohio, USA. mandibles would further comminute the shell material. Shells would be SDSMT: Museum of Geology, South Dakota School of Mines and shattered as a result, and we hypothesize that these fragments would be Technology, Rapid City, South Dakota, USA. indistinguishable from shell material that had been broken up by USNM: United States National Museum of Natural History, abiotic processes. Smithsonian Institution, Washington, D.C., USA. Oji et al. (2003), however, suggested that shells broken as a result of UT: University of Texas at Austin, Non-Vertebrate Paleontology durophagous predation might be recognizable. Studies of foregut Lab, Austin, Texas, USA. contents of Metanephrops